Patent GB-824336-A (GB)
Title: Improvements relating to automatic telephone and like selective systems
Inventor: WARREN BLOOMFIELD JAMES
IPC: H04Q3/00
Filed: 1956-08-02
Abstract: 824,336. Automatic exchange systems. SIEMENS EDISON SWAN Ltd. July 16, 1957 [Aug. 2, 1956], No. 23884/56. Class 40 (4). In a system of the type in which a selector is controlled by impulses which it reverts to a control circuit, spurious impulses caused by contact bounce are rendered ineffective by connecting the wiper of the selector to a transistor trigger circuit which changes from one stable state to the other only when the wiper moves from a contact having one potential to a contact having another distinct potential, the output of the trigger circuit being employed to step the impulse counter in the control circuit. As shown, two junction transistors T1, T2 have their emitter electrodes connected through a common resistor R to a point P of fixed potential, e.g. ground. The collector electrodes of T1 and T2 are respectively connected to the junctions of R1 and R2, and R1<1> and R2<1>, in the two chains of resistors R1, R2, R3 and R1<1>, R2<1>, R3<1>; and the base electrodes to the junctions of R2<1> and R3<1>, and R2 and R3. The lower end of each chain is connected to the fixed potential, e.g. ground, and between the upper ends is connected the primary winding of an output pulse transformer PT, the centre-tap of which is connected to a point N having a negative potential -V. The base electrode of T1 is also connected via resistor R4 to the bridging wiper W of the selector S, some of the bank contacts of which are connected to the fixed (ground) potential, and the others of which are connected to the relatively negative potential -V. If the selector S is a line-finder or a final selector the oddnumbered contacts may be connected to -V and the even-numbered ones to ground, whereas if it is a group selector the so-called pilot contacts at the commencement of each group of outlets may be connected to -V and the remaining contacts to ground. Assuming that the wiper W starts on a grounded contact, the transistor T1 will be held non-conductive since its base potential is positive to that of its negative emitter, the other transistor T2 being conductive. On reaching a contact connected to -V the wiper W first bridges this contact and the preceding grounded contact thus V applying to the base of T1 the mean (- -) of 2 the two contact potentials. The values of the resistors R1, R2, R3, R1<1>, R2<1> and R3<1> are so chosen that this mean potential does not make the base of T1 negative with respect to its emitter, so that T1 remains non-conductive during the bridging period. However, when the wiper W engages only the contact connected to -V, T1 conducts and T2 is rendered non- conductive this change-over producing a pulse in the primary winding of transformer PT. Similarly, when the wiper moves on to a grounded contact the mean potential applied to the base of T1 during the bridging period is not sufficiently positive to stop T1 conducting, but when the wiper leaves the negative contact T1 is rendered non-conductive and T2 conducts, the changeover producing a pulse in PT as before. Depending upon whether an output pulse is to be transmitted to the control circuit (not shown) on each change of the conductive states, or only on alternate changes, the secondary winding of PT is connected through either a full-wave (as shown) or a half-wave rectifier arrangement to an output terminal O. The output pulses may be clamped to a suitable valve by a catching rectifier such as Rf5. As the trigger circuit also provides amplification, pulses of lower power may be reverted from the selector, thus allowing an improvement in the level of cross-talk between switching paths to be achieved.